Optical recording media such as compact disks (CDs) have become very popular as a means for storing and/or reproducing information such as audio information, video information or other data. CDs with one information surface usually include, for example, a spiral or segmented information track composed of convex and concave portions (pits) formed on the surface of a disk-shaped resin substrate comprising, for example, polymethylmethacrylates and polycarbonates. A reflection film of, for example, aluminum (Al) is deposited on the information bearing surface of the substrate and a protection film is formed thereon.
The storage capacity of such a CD is too small to store, for example, moving-picture information such as a movie for a sufficiently long recording or play time. In order to increase the storage capacity, storage media having two disks which are adhered together, such as, for example, magneto-optical disks (MO) have been suggested.
Recently, a high storage capacity format for digital video storage has been suggested as DVD (digital versatile disk, sometimes also digital video disk). DVDs can be divided into DVD-ROMs which are exclusively for reproduction, and DVD-RAMs which can also be used for data storage, DVD-Rs which are recordable once ("write once, read many"), DVD-Audios and DVD-Videos. The format for DVD-ROMs, for example, has been provisionally defined by an industry (ECMA, Executive Committee of the DVD Consortium) as ECMA/TC 31/97/2, Geneva, Switzerland, January 1997, and presently comprises 4.7 gigabytes (GB), 8.5 GB, 9.4 GB and 17 GB formats. The different formats are described in some detail on p. 5 of ECMA/TC 31/9712 as types A-D and in EP 0,725,396 (see, in particular, FIG. IC-IF).
DVDs generally comprise two disk shaped members, the inner surfaces of which are bonded together with a bonding layer which must be light transmissive if the information on one or more of the recording layers is read through the bonding layer. In the 4.7 GB single layer format only one of the members comprises an information storing layer formed by the inner surface of the substrate which exhibits a sequence of pits with a minimum size of, for example, 0.4 .mu.m. An Al layer is deposited on the information storing layer optionally followed by a protective coat. The inner surface of the second substrate which is a dummy substrate is bonded to the inner surface of the first member, i.e., for example, to the Al layer or to the protective coat layer. The 8.5 GB and, in particular, the 9.4 GB and 17 GB formats exhibit more complicated constructions and usually comprise an information storing layer on each substrate. For the 8.5 GB format a single side/dual layer construction is described on p. 3, FIG. 3 of the Bulletin: 101 December 1996 provided by Imation Corporation of Oakdale, Minn., available on the Internet under the address http://www.imation.com/dsp/optical/dvd/techbull.html.
Bonding of the two members of DVDs or other high storage capacity optical recording media is a particular problem because of the demanding technical requirements. In Tape-Disk Business, 10 (1996), no. 9, p.13, three bonding technologies are described using hot-melt adhesives, radically ultraviolet (UV) curable or cationically UV curable liquid adhesives, respectively. Hot-melt adhesives can be attached to the inner surface of the members, for example, by roll-coating whereas radically or cationically UV curable liquid adhesives can be applied with, for example, spin-coating or screen-printing techniques.
While UV-curable liquid adhesives may result in bondings having a low defect density or in a virtually "perfect bubble-free system" as is claimed in the Tape-Disk Business article, their use also has some disadvantages such as, for example, corrosion problems of the members and, in particular, of the reflective films used herein, the incompatibility of liquid adhesives with mass production requirements and handling problems as is pointed out in the background section of EP 0,330,197. Additionally, UV curing is usually difficult because of the high UV-absorbance of the substrates. This necessitates high UV intensities and/or long times for curing resulting in unacceptably high tilt or warpage. UV-curing at the edges of the members where oxygen is present, is difficult. EP 0,330,197 suggests the use of a smooth-surfaced silicone-based double-side pressure-sensitive adhesive film which is applied to the inner surface of the first disk, followed by pressurizing the first disk with the attached adhesive film between rollers, removing the release liner, attaching the second disk to the exposed pressure-sensitive adhesive film, and application of uniform pressure. A similar process is described in M. Nakamura et al., Two-sided tape used to glue DVDs together, Japanese Journal of Electronic Materials (Denshi Zairoy), special edition June 1996, pp. 46-49. In a first step the first member is secured with the first reflective layer being exposed, and the adhesive sheet is then adhered to the first substrate using a rubber roller. Then the release liner is removed from the adhesive layer, the inner surface of the second member is attached to the adhesive sheet and the assembly is then press-bonded in a pressure boiler to expel the bubbles which have been formed during the lamination step. Expelling the bubbles requires that the second substrate is pushed down towards the exposed adhesive layer adhered to the inner surface of the first substrate using, for example, a pressure cylinder exerting high pressure. This often results, however, in the formation of large bubbles and/or an irreversible deformation of the adhesive layer near the bubble. Another problem is that the pressure sensitive adhesive layer is squeezed out between the edges of the members. The surface of the members typically is not essentially flat but may comprise protrusions or indentations. Disk-shaped members with a centered hole, for example, typically exhibit a rim around said centered hole to allow for stacking the optical recording medium on a vertically extending pin. When quickly pushing down the second member by means of a piston, for example, any mismatch in the orientation between the piston and the second member may result in introducing high tilt or warpage and/or scratches on the surface of the member. When using a piston it is furthermore difficult to bond the members together without creating unacceptable high values of tilt and/or warpage in the DVD. Pushing down the second member onto the exposed adhesive layer and expelling the air is required in the method described in the above article from the Japanese Journal of Electronic Materials to obtain cycle times of no more than 10 seconds per DVD.
The use of a double-sided adhesive layer in DVDs is also mentioned, for example, in JP 07-262,619, JP 07-014,215 and JP 08-096,415.
It is seen that it is desirable to provide an apparatus and method for applying a bonding layer to a member of an optical recording medium that minimizes or eliminates entrapped bubbles and/or that provides a flat member. It is also desirable to provide an apparatus and method for assembling two members of an optical recording medium that minimizes or eliminates entrapped bubbles and/or that provides a flat medium. Preferably, such an apparatus and method is particularly well suited for use with pressure sensitive adhesives, although it is not limited thereto.